The present invention relates generally to spectrometers, and more particularly, to self-calibrating spectrometers.
Typically, in order to calibrate a spectrometer, a reference sample having known reflection or transmission characteristics must be measured. Subsequently upon measuring a sample of unknown reflection or transmission, the spectrometer will compare the signal obtained using the unknown sample to the signal obtained using the reference and then will calculate the absolute reflection or transmission from this ratio.
If the conditions present at the time of reference measurement do not change, then the calibration is valid and the measurements on the unknown samples will be accurate. This is an ideal case.
However, some of the parameters that will cause the system response of the spectrometer to change with time include (1) lamp intensity changes, since all lamps burn down or degrade, (2) optical fibers can be inadvertently re-positioned or bumped, (3) at UV wavelengths, fiber-optic materials solarize, causing the transmission of the fibers to degrade with time, and (4) dust can collect on system elements causing the transmission of the spectrometer to degrade. Consequently, it would be desirable to have a spectrometer that is self calibrating.
Many spectrometers are designed so that a reference sample is measured every time an unknown sample is measured. A dual beam spectrophotometer is a good example of such an instrument. However, this type of instrument is large and expensive, and as such, it is not practical for certain applications such as in-situ monitoring, where it is desired to monitor the etch or deposition of coatings inside of a process chamber. This system also has multiple moving parts that make the system more susceptible to mechanical failure.
It is therefore an objective of the present invention to provide for self-calibrating spectrometers.
To accomplish the above and other objectives, the present invention provides for auto-calibrating spectrometers and methods that measure transmission or reflection versus wavelength of a sample without need for calibration for long periods of time, up to 15 days or more. Reflection and transmission spectrometers along with auto-calibrating methods for use therewith are disclosed. Light is focused onto a sample using a lens or similar optical element that transmits light towards the sample reflects light impinging upon it, and transmits light reflected from the sample. If one monitors the light reflected from the first lens and sample, very useful information is available related to the system response versus time. The present invention monitors the reflected light from the first lens and sample, and corrects for the system changes over time using this reflected light.
An exemplary reflection spectrometer comprises a light source, an optical element that transmits light and reflects a small amount of light, and a detector for outputting electrical signals corresponding to light signals that are detected thereby. Optical coupling apparatus, such as a fiber optic cable, or lens and beam splitter combination, couples light from the light source to the optical element. The fiber optic cable preferably comprises at least one illumination fiber for coupling light to a sample under measurement and a detector fiber that collects light reflected from the optical element and directs it to the detector. A shutter assembly may be used to selectively couple light or inhibit light from impinging upon and reflected by a reference sample having known reflection or the sample under measurement. A controller is coupled to the detector that processes the electrical signals output thereby and implements an algorithm that calculates a calibration value for the spectrometer at each wavelength of light output by the light source using a predetermined equation to autocalibrate the spectrometer.
An exemplary transmission spectrometer further includes a second focusing lens for receiving light that is transmitted by or reflected off of the sample under measurement toward it, a second detector coupled to the controller, and a second fiber optic cable for coupling light received by the second focusing lens to the second detector. In the transmission spectrometer, the controller processes the electrical signals output by both detectors and implements an algorithm that calculates a calibration value for the spectrometer at each wavelength of light output by the light source using a second predetermined equation to autocalibrate the spectrometer.
Important aspects of the present spectrometers include a low manufacturing cost and self calibration. Also, in a reduced-to-practice embodiment, there are only two moving parts (shutters) that in several configurations or applications, are not required. Furthermore, the spectrometers are ideally suited for in-situ monitoring applications.
An exemplary auto-calibrating method for use with a reflection spectrometer comprises the following steps. An initial calibration of the spectrometer is performed. A background scan is performed with the light source on and the shutter assembly closed. A background scan is performed, if required, with the light source off and the shutter assembly open. A background scan is performed with the light source off and the shutter assembly closed. A sample scan of the sample under measurement is performed with the light source on and shutter assembly open.
An exemplary auto-calibrating method for use with a transmission spectrometer comprises the following steps. An initial calibration of the spectrometer. A background scan of the first and second detectors is performed with the light source on and the shutter assembly closed. A background scan is performed, if required, of the first and second detectors with the light source off and the shutter assembly open. A background scan is performed, if required, of the first and second detectors with the light source off and the shutter assembly closed. A sample scan is performed using the first detector of an unknown sample with the light source on and shutter assembly open.